Study Finds Global Warming Will Stress Region’s Forests

Leading University of Montana researchers have released results of a new study that shows climate change will increase drought stress in northern Rocky Mountain forests, leading to increased potential for insect infestations and risk of more frequent and severe wildfires.

Missoula, Montana – Jan 5, 2010

The peer-reviewed study, conducted by UM forestry researchers, finds that longer periods of drought will stress the forest ecosystem that includes areas in Montana, Wyoming and Idaho, with increased insect epidemic and wildfire disturbances. The economic impact of highest concern is the potential of a catastrophic wildfire in the region, which could affect more than 360,000 people who live in homes in the forest-urban interface that are valued at $21 billion.

“As temperatures rise, we will see about two months of additional drought stress each year by late this century,” said study author Steve Running, Regents Professor of Ecology in UM’s College of Forestry and Conservation. “And the worse global warming gets, the more significant the consequences for forests.”

Key findings of the study include:

* As temperatures rise, projected changes in northern Rocky Mountain forests include fewer days with snow on the ground, earlier peak snowmelt, a longer growing season and about two months of additional ecosystem drought stress each year by late this century. * Increasing drought stress will increase forest disturbances, including insect epidemics and wildfires. These disturbances have large impacts on society and the natural world. * The economic impact of highest concern is the potential for a truly catastrophic wildfire in the region. There are now 360,000 people living in homes valued at $21 billion in the forest-urban interface in this region that are directly vulnerable to wildfire. * If climate becomes drier, net carbon uptake would be reduced to the extent that most forests in the region would switch from absorbing carbon to releasing it by late this century.

“Global warming will cause the spring snowmelt to occur four to six weeks earlier and the summer drought period to be six to eight weeks longer,” Running said. “By the 2080s, these dramatic shifts will leave the forests stressed and increasingly vulnerable to insect infestation and wildfire.”

The new study shows that the forests of the U.S. northern Rocky Mountains are highly sensitive to projected climate change. Even under conservative projections of future climate change, Running said dramatic effects on these forests are expected.

Documented climatic changes in the last 50 years have significantly altered the conditions in which forests grow, and the research shows that forests already are responding to observed climate change. These changes are projected to intensify in the coming years. Forests in relatively dry regions such as the northern Rocky Mountains live in a perpetually water-limited state, Running said. During most of the growing season, when light and low temperatures do not limit growth, water is the most important limiting factor. Productivity depends on moisture conditions during the main part of the growing season. Spring, summer and autumn temperatures, summer precipitation levels, and the previous winter’s snowpack determine those moisture conditions.

Climate projections for the northern Rocky Mountains over the course of this century include an annual average warming trend of 3.6 to 7.2 degrees Fahrenheit. Lower emissions of heat-trapping gases will result in temperature increases near the lower end of this range and higher emissions near the higher end. Running said winter temperatures are projected to increase more than those in the other seasons. Precipitation, runoff and stream-flow patterns also will change, with both the amounts of water and the timing of runoff and stream flow being affected.

Over the course of this century, the growing season in the northern Rocky Mountains is expected to increase by about two months, Running said. The growing season will shift one to two months earlier in the spring. Late summer drought will be extended by six to eight weeks. One of the results of this extended drought will be an increased risk that small streams will dry up.

The study was funded by the National Commission on Energy Policy, a bipartisan, nonprofit organization that examines key policy issues related to energy.